This invention relates to a fluidized bed combustion system and, more particularly, to such a system including a reactor connected to a separator in which a duct connecting the reactor and separator extends within the furnace section in the reactor.
Fluidized bed combustion systems in which reactors are used in connection with separators are well known. In these arrangements, a bed of particulate fuel, usually in the form of coal, wood or dehydrated sewage sludge, is provided in a furnace section of the reactor. Air is passed through the bed of particulate fuel to fluidize the bed in the furnace section, and thereby, effectuate high combustion efficiency at a relatively low temperature. This process, however, results in flue gases which entrain a large amount of fine particulates. The flue gases are therefore passed into a separator which separates the particulates from the flue gases and recycles the separated particulates back to the bed in the furnace section.
Fluidized bed combustion systems generally work well and have several advantages, such as permitting efficient fuel combustion and use while maintaining low emission levels for pollutants such as NO.sub.x and SO.sub.x. However, these systems are not without problems. For example, separator efficiency and thermal stresses in the system require the use of complex and costly means for connecting the reactor with the separator, including the use of costly expansion joints and seals.
Additionally in conventional fluidized be combustion systems, the passage between the reactor and the separator is usually defined by a relatively expensive, high temperature, refractory-lined duct due to the extreme temperature of the flue gases. This duct is either made relatively thin due to the expense and weight of the refractory material which results in excessive heat losses to the environment, thereby reducing the system's efficiency, or it is made relatively thick which adds to the bulk, weight and cost of the separator. Even when the duct is thick, all the heat losses cannot be prevented since perfect insulation would raise the duct's temperature to an unacceptable degree.
A further problem associated with the use of a refractory-lined duct is the lengthy time required to warm the walls before putting the system on line to eliminate premature cracking of the refractory material. This lengthy delay is inconvenient and adds to the cost of the process.